1. Field of the Invention
The present invention relates to power line communications (PLC), and more particularly, to a configuration of a modem having an optimized output power level over its operating frequency spectrum.
2. Description of the Related Art
In a power line communication (PLC) system, output power of a transmitter in a wideband power line modem is coupled to a power line. The power line has an impedance magnitude characteristic and a phase characteristic that vary widely across a frequency band of the transmitter. The transmitter can be modeled as a Thevenin equivalent circuit having a fixed voltage source and a mainly resistive source impedance. As a result of an impedance mismatch between source and load, signal power coupled to the power line varies widely over the transmitter frequency band.
When the signal power level is limited to a highest level compliant with regulatory limits, the limit is set for a frequency where power coupling and power line radiation efficiency are most efficient. This frequency is referred to herein as fmax-rad.
FIG. 1 is a block diagram of a prior art modem arrangement with a flat transmitter spectrum. A modem 100 is attached to a power line 120 via a signal coupler 125. A functional internal structure of modem 100 is represented by a modem transmitter signal generator 105, a power control system 110, and a power amplifier 115. Signal generator 105 and power control system 110 may be physically embodied in the same module of hardware and software, but are illustrated here separately for clarity.
Graph 150 shows a simplified variation of electromagnetic radiation intensity for modem 100. Modem power 155 is constant across a frequency band and is limited to Pmax1 160 to prevent radiation intensity 162 from exceeding radiation limit 165 at frequency fmax-rad 170. However, at other frequencies, the coupled power will be lower than that allowed by regulatory limits, and thus, performance of modem 100 is less than optimal.